Synesthesia is a fascinating condition in which stimulation of one sensory modality causes experiences in a second modality. For example, when presented with musical tones, some people report seeing colors in addition to hearing those tones. Estimates of the prevalence of synesthesia range from 1 in 20 (Galton, 1880/1997) to 1 in 20,000 (Cytowic, 1989; Cytowic, 1997). We will use a combination of perceptual experiments and functional magnetic resonance imaging (fMRI) to explore the neural basis of synesthesia. We propose that number-color synesthesia is a result of neural "miswiring" between areas of the fusiform gyrus involved with number-form processing (Richard, Romero, Basso, Wharton, Flitman, & Grafman, 2000; Pesenti, Thioux, Seron & De Volder, 2000) and more interior areas of the fusiform gyrus considered to be involved with processing of color (Lueck, Zeki, Friston, Deiber, Cope, Cunningham, Lammertsma, Kennard & Frackowiak, 1989; Zeki & Marini, 1998). Should this hypothesis be borne out, it would provide an existence proof of the neural miswiring hypothesis and could lead to further studies investigating other forms of synesthesia, including tone-color synesthesia and Glaton's (1880/1997) "number-forms."